The results of a 5-year study of adolescents in The Netherlands shows that young people with a certain gene variant are more vulnerable to depression when they smoke pot.

Until now, the link between smoking marijuana and depression was suspected, but evidence was weak. Roy Otten at the Behavioural Science Institute of Radboud at Nijmegen University has published the first definitive association between smoking herb and depression, but specifically in young people with a specific genetic variant. He published his work in the journal Addiction Biology.

Otten and his colleagues collected data from 310 adolescents over four years. Each year the kids answered questions about their behavior and mood. The researchers also tested for a serotonin gene called 5-HTT, which is associated with increased vulnerability to developing depression. Then they also collected data from the group's younger siblings and saw the same results.

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Otten says the results are robust and that he took into consideration other factors that could have contributed to depression, including alcohol use, upbringing, personality and socio-economic status. He also accounted for the possibility that the depression came first, and that the study subjects were self medicating. "In the longer term, that is definitely not the case," Otten said it a press release.

Stay tuned to find out whether these results will be duplicated in future studies, and whether the same will be true for adults. Perhaps individual genetics are another reason why the effects of smoking marijuana are so unpredictable.